Add Data Sharing Agreement

[dave-mills]

Each institution must have agreed to the data sharing agreement to use the tool. This agreement can be made on behalf of an institution by any institutional admin.

• The agreement (text still to be written) must be available on the institution's main page.
• If the agreement is not digitally signed on behalf of an organisation users will not be able to create or assess inititatives (we should prevent access to the relevant pages and provide a sensible error message explaining what users need to do).
• Institutional admins (and only admins) should have the ability to digitally sign the agreement by ticking a box on the institutional settings page.
• For institutions who have not yet signed, there should be a constantly visible warning notification visible just above the main page contents on the "My Institutions" page.

[dave-mills]

This should be similar to our existing setup on the Fall Armyworm data platform, but more strict i.e. it actually prevents key tasks on the platform until the agreement is signed.